This invention relates to apparatus for maintaining the desired temperature of a fluid, and more particularly, is shown as an example of its use, as embodied in a heat exchanger for reducing and maintaining the temperature of a fluid to a desired level. Specifically, the present invention relates to a wet surface, air cooled heat exchanger wherein an air moving means induces an air flow over a heat exchange element such as a tube bundle or coil through which the fluid to be cooled passes.
An air cooled heat exchanger consists generally of a tubular heat transfer surface and a fan. The fluid passing through the tubes surrenders heat to air propelled by the fan over the external tube surface. The heat energy of air passing over the tubes increases in the same amount as the heat energy loss by the fluid, when assuming that no other heat losses occur. As known to those skilled in the art, a reduction of the air mass passing over the heat transfer surface will reduce the amount of absorbed heat by a proportionate amount. Therefore, the cooling capacity can be controlled by varying the air mass passing over the tubular heat transfer surface.
It is known to vary the air mass passing over a heat transfer surface by, for example, reducing the speed of an air moving fan or by decreasing the pitch of the propeller blades of a fan, if propeller fans are utilized. The air mass flow could also be throttled by a damper, for example.
It is also possible to vary the heat transmission rate from the heat transfer surface by returning a portion of the exhausted air which has passed over the heat transfer surface to the heat transfer surface again, while commensurately reducing the air entering from the outdoors, for example, by the amount of air which is returned or recirculated. A reduction in the recirculated air, as well known in the art, will increase the amount of heat surrendered to the air in a proportionate manner. Accordingly, the cooling capacity can be also controlled by varying the amount of recirculated exhaust air, while keeping the air mass passing over the heat transfer surface generally constant. This method is known as capacity control by proportioned air recirculation.
In order to achieve capacity control by proportioned air recirculation, known practices require a system of coordinated dampers to vary the amount of air to be exhausted, the amount of air to be recirculated and the amount of fresh outside air to be combined with the recirculated air for passage over the heat transfer surface. Of course, the use of multiple dampers results in considerable air turbulence and a resulting reduction in efficiency of the entire apparatus.
U.S. Pat. Nos. 2,296,946; 2,321,933 and 2,445,199 are exemplary of recirculation arrangements including multiple dampers. The dampers at the fresh air inlet and exhaust outlet ports must function in unison and their positions must be inversely proportional to the position of the damper at the recirculation port, the dampers in all three ports to open and close in a coordinated manner. Thus, if the fresh air inlet and exhaust outlet were wide open, the air return or recirculation means would be closed and, of course, if the air inlet and exhaust openings were closed, the recirculation duct would be open. The present invention is concerned with the use of a single damper blade in lieu of multiple damper blades to achieve proportioned air recirculation, with greater simplicity of design and dependability of operation.
The prior art has also considered various means or, more specifically, linkages for operably positioning multiple damper assemblies and single damper assemblies, an example of the latter being disclosed in U.S. Pat. No. 2,379,932. All of the dampers used in the type of apparatus referred to are subjected to various degrees of buffeting by the air impinging thereagainst. The configuration of the damper disclosed by the present invention substantially reduces buffeting and corresponding turbulence. Most importantly, this damper will be entirely out of the air stream whenever all air is exhausted for maximum capacity. In this regard, the present invention is also concerned with a unique, pneumatically actuated linkage system for positioning the damper of the present invention.
Another substantial problem encountered in prior art air cooled heat exchangers involves the fact that the same discharge air upwardly into the atmosphere to dissipate the absorbed heat. This practice has been undertaken with a view towards allowing the heat-laden air stream to follow a natural updraft path. However, in order to recirculate a portion of the exhaust or discharge air, it must then be channelled from a point above the heat transfer surface to a point below it. With the admission of fresh outside air below the heat transfer surface, the same must be proportionately reduced by a damper in the same measure as the amount of recirculation air increases. Necessarily, a failure to do so would cause excess outside air to be admitted below the tubular surface by a natural updraft of air past such surface, possibly aided by wind blowing against the fresh air inlet opening. Such excess outside air would then escape at the air discharge. The updraft of excess outside air past the heat transfer surface is particularly undesirable if no cooling is required, and the air moving means has been stopped. For example, such exposure to a potential updraft of outside air would increase the possibility of damage from freezing. Therefore, known practices placing the heat transfer surfaces in a natural updraft depend on a damper at the fresh air inlet below the heat transfer surface. This measure has only a limited effectiveness since the natural air updraft promotes the entrance of outside air by leakage.
Furthermore, it is known practice to discontinue wetting the heat transfer surface in cold weather when all air passing over the heat transfer surface is recirculated, and the air moving means is stopped. However, such periodic interruption of wetting the heat transfer surface promotes the formation of scale as the minerals dissolved in the wetting water dry on the metallic heat transfer surface.
In contrast, the disclosed arrangement, having only a single damper blade to recirculate a portion or all of the exhaust air does not promote the passage of cold outside air over the heat transfer surface by a natural updraft and does not require an additional coordinated damper at a fresh outside air inlet. In addition, a massive drenching of the heat transfer surface does not have to be interrupted in sub-freezing weather.
The present invention is also concerned with the use of a propeller fan as an air mover for recirculated and fresh outside air passing over a heat transfer surface. It is known in the prior art to utilize propeller fans generally for this purpose as, for example, illustrated in U.S. Pat. No. 2,379,932 to Schoepflin et al. However, in the Schoepflin et al. arrangement, the heat transfer surface is disposed immediately downstream of a propeller fan so as to disturb the normal flow pattern of air from the propeller fan--the flow of air discharged by a propeller fan being normally directed towards the fan axis. Necessarily, such disturbance to the propeller air flow presents a substantial inefficiency. Such inefficiency is of increasing concern when large air volumes are involved.